1. Field of the Invention
The invention relates to mechanisms of contractility, and to systems utilizing "McKibben" muscles.
2. Description of the Prior Art
Contractility describes the fundamental mechanism by which motion or movement is accomplished by living matter. Contractility is thought to result from the interaction of various fibrillar (contractile) proteins within the living cell. The contractile properties of such proteins are thought to primarily result from chemical interactions. Accordingly, investigations of the contractile properties of muscle cells primarily focus on the chemical aspects (reactions), rather than the mechanical aspects of contraction.
It is generally believed that two types of filament structures form the contractile machine in a muscle, ie, a "thick" filament composed primarily of myosin and a "thin" filament which contains actin. It has been discovered that the filaments, whether thick or thin, do not change in length as a muscle cell contracts from its relaxed state. It is also known that "thin" filaments are attached at either end of the muscle cell and that the "thick" filaments bind to the "thin" filaments. It has been hypothesized that the "thin" filaments mechanically slide by the "thick" filaments.
The magnitude of the change in the overall length of the muscle cell during contraction is typically less than 3 microns. In fact the sarcomere length of some skeletal muscles is only about 2.5 microns indicating that such skeletal muscles do not signifigantly shorten during contraction.
Summarizing, muscle cells and muscles have the capability of producing very large forces upon contraction. Muscles also tend to increase in diameter as they contract. However, the mechanical aspects of the contractility mechanism of a muscle cell are not well understood.
Mechanical systems simulating the action of a muscle described in the literature typically include an expandable bladder confined and/or constrained to expand radially when pressurized with a gas. For example, a Brevet D' Invention published Oct. 15, 1971 in the Republic of France, No. 2.076.768 describes a device for converting fluid pressure into contractile force and includes an ovoidal rubber envelope reinforced with relatively inelastic longitudinal filaments. The ends of the ovoidal envelope are anchored between two points and the envelope pressurized with a gaseous fluid causing the ovoidal envelope to expand diametrically and contract longitudinally pulling the anchor points together. Similarly, U.S. Pat. No. 3,645,173, Yarlott, describes an ovoidal envelope defined by an expandable bladder confined within relatively inelastic longitudinal strands such that the bladder can expand diametrically but not longitudinally when pressurized with a gaseous fluid.
The earliest known mechanical system which simulated the action of a muscle known as the "McKibben Synthetic Muscle" comprises a cylindrical sheath woven from helical strands of an inelastic material surrounding a gas inflatable bladder, and was designed to articulate orthopedic and prosthetic appliances for polio victims.
While gas has advantages as a medium for pressurizing and expanding "McKibben" type contractile actuators or muscles, its primary disadvantage is that it is compressible which dictates an "elastic" contractile response. Elastic contractile responses are not always desirable. For example, an elastic contractile mechanism can not establish or maintain a reference position where load varies.
Also, as explained infra, the contractile force generated by a "McKibben" type contractile device as it inflates is not linear, and when a gas is utilized to inflate or "energize" the muscle, the contractile position of the muscle will vary with gas pressure and tensile load. Accordingly, any system utilizing gas inflatable "McKibben" type contractile devices requires compelx electromechanical or other type of servomechanisms both for adjusting position, and for metering the gas for inflating and deflating under conditions of varying pressure.
Still another significant disadvantage of using a gas for inflating and contracting a "McKibben" muscle relates to the property of gas to expand filling the available volume. This means that the degree of contraction (or extension) of the muscle is determined by the tensile force between the anchor points of the muscle resisting contraction.
The above reasons, among others have discouraged commercial acceptance of the "McKibben" muscle as a contractile force generating mechanism/actuator.
Summarizing, gas energized contractile force mechanisms have three variables, position (volume), pressure and tension. Temperature also affects the response of such gas energized systems. For the "McKibben" type contractile force mechanisms to achieve commercial acceptance, it is necessary to eliminate or minimize the above variable affecting its response.